1. Field of the Invention
The present invention relates to semiconductor processing technology and, in particular, concerns a method of cleaning and monitoring pads used in planarizing of the surfaces of a wafer using chemical mechanical polishing.
2. Description of the Related Art
Chemical mechanical polishing or planarization (CMP) is a technique whereby surfaces, such as semiconductor substrates, are planarized by the simultaneous application of both etching and polishing processes. CMP is typically used to globally planarize surfaces such as the upper surface of a semiconductor wafer. The wafer is typically positioned within a carriage and is rotated with respect to a polishing pad. In one approach, a slurry containing abrasive particles and an etchant is interposed between the polishing pad and the surface of the semiconductor wafer that is to be planarized. The polishing pad is then brought into contact with the surface of the wafer that is to be planarized and the combination of the mechanical polishing and the etchant results in the exposed surfaces of the wafer being removed by the CMP process.
One specific technique of CMP, known as copper CMP (CuCMP), is used to remove copper (Cu) from the wafer surface. CuCMP is presently used extensively in conjunction with a copper application technique known as copper Damascene process. One method of laying metal lines and interconnects in the integrated circuits is to form a layer of metal on the wafer and chemically etch away the metal. Copper is the metal of choice over other metals such as aluminum and tungsten, due to its desirable electrical properties. Copper, however, is difficult to use in the etching technique due to its high susceptibility to corrosion during the process. Corrosion leads to unpredictable electrical properties of the resulting copper interconnects, thus making copper essentially unusable for such an application. The copper Damascene process overcomes the corrosion problem by depositing copper directly into the groove patterns of interconnects already formed within the dielectric layer of the wafer. Since the copper is not etched away chemically, copper corrosion is no longer a problem. The excess copper from the Damascene process is removed by CuCMP.
As any material is removed globally from the surface of the wafer, it is desirable to be able to stop the CMP process after a predetermined amount has been removed. Endpoint technique is a method of stopping the CMP process after a right amount of material has been removed. Typically, endpoint techniques rely on frictional properties and/or light reflecting properties of the surfaces involved in the CMP process. As a layer of material is being removed from the wafer, that layer exhibits certain friction and reflectivity. When that layer is polished off and a new layer is exposed, friction between the pad and the wafer surface changes. Also, the reflectivity of the surface changes when the new layer is exposed. The CMP system can detect either or both of these changes and establish an endpoint. One of the parameters that aids in accurate endpoint technique is the removal rate that depends on the condition of the polishing pad.
One of the problems associated with the CuCMP is that slurries used in the CuCMP process are highly reactive with copper, and the various copper byproducts end up being lodged in the pad. As the copper is removed, copper byproducts are formed and begin to clog the pores and grooves on the pad. As the pores and the grooves get clogged, the slurry cannot flow uniformly throughout the surface of the pad, and glazing may occur at various locations, thus causing a non-uniform removal rate of the pad. To overcome this problem, pads are typically cleaned prior to use on a wafer.
Pad cleaning involves restoring the surface of the pad followed by chemically rinsing away the copper byproducts from the pad. The surface is restored typically by using a diamond grinding disk that comes into contact with the pad in a manner similar to that of the silicon wafer being planarized. The abrasive diamond grinding disk breaks up any glaze that may have formed on the pad's surface, and also restores a desired roughness of the pad's surface. Once the surface is mechanically restored, residual particles and the copper byproducts from the pores and grooves are dissolved away using a rinse solution. A typical rinse solution comprises a 5% ammonium citrate solution.
Despite cleaning prior to each use, pads used in CuCMP still show drifts in removal rate, and recent data show that amount of copper byproducts absorbed in the pad increases over time as the pad is cycled between cleanings and uses. Some of the methods used to measure such data are disclosed in technical publications such as “Cu dissolution from Si(111) into an SC-1 process solution”, D. Chopra et al., Journal of Electrochemical Society, Vol. 145, No. 4, 1998, and “An optical method for monitoring metal contamination during aqueous processing of silicon wafers”, D. Chopra et al., Journal of Electrochemical Society, Vol. 145, No. 5, 1998. Such measurements indicate that the present method of cleaning of pad does not remove the copper byproducts sufficiently. Furthermore, a fabricator using a typical conventional pad cleaning method does not know the actual condition of the pad.
While the current method of cleaning the pads for use in CuCMP process does remove copper byproducts, it is desirable that there be a more consistent method of cleaning and monitoring the pad. In particular, it is desirable to have a method of determining the concentration of the copper byproducts lodged in the pad accurately so that a fabricator can better understand the cleaning process so as to form an endpoint technique in the cleaning process. Additionally, it is desirable to map out the condition of the entire pad boundary so as to be able to achieve uniform cleaning that will lead to uniform removal of material from the wafer. By knowing the copper byproduct concentration over the entire boundary of a given pad, a proper cleaning and a proper endpoint technique can be worked out for that particular pad to yield a predictable and uniform removal rate, thus yielding a higher quality planarized wafer.